One XQD slot is better than 2 SD slots. PERIOD. You're more likely to have TWO SD card failures in the Sony than a single XQD failure in the Nikon.
Nikon has the better mousetrap....better LCD, viewfinder, faster FPS, better video, ergonomics and likely better IQ.
The Z6 is an amazing start, but some credit has to go to Sony for taking many faltering, but important steps that allow Nikon and Canon to roar into the mirrorless market.
And again, I've seen TWO A7rIII cameras fail in the field. I've seen SD cards fail, especially after rough handling. But XQD? I have to google it to find anyone with issues. Of course we have trolls claiming their XQD's failed and they know people who's had issues blah blah blah. XQD is rock solid and ultra reliable. Period.
I'm holding off on the Z6 to see detailed reviews, but can't see why I won't be buying one to use alongside by D850. The Z6 is a beautiful camera.
Rob